IP multimedia core network subsystem (IMS) is an IP-based network architecture proposed by the 3rd generation partnership project (3GPP), and it constructs an open and flexible service environment to support multimedia applications and provides users with enriched multimedia services.
In the IMS service system, the control layer and the service layer are with separated, and the control layer only provides the service layer with necessary functions such as triggering, routing and charging rather than specific services.
The functions of service triggering and controlling in the control layer are implemented by the call session control function (CSCF). There are three kinds of CSCFs: proxy-CSCF (P-CSCF), Interrogating-CSCF (I-CSCF) and Serving-CSCF (S-CSCF), wherein S-CSCF takes the main responsibility, and the I-CSCF is an optional network element.
The service layer consists of a series of application servers (AS) to provide specific services, and the AS can be an individual entity or exist in the S-CSCF.
According to the user's contract information, the S-CSCF of the control layer controls the service triggering, calls the services in the AS, and implements the service function. The AS and S-CSCF can be collectively called service equipment (SE).
The end-to-end equipment in the session is called user equipment (UE) and is in charge of the interaction with the users; some UEs can access the network by a variety of methods including accessing the network via the 3GPP packet switch (PS) domain, accessing the network via other non-3GPP PS domain and even accessing the network via the circuit switch (CS) domain.
If the CS network is configured with an enhanced mobile switch center (eMSC), the eMSC provides a session initial protocol (SIP) interface to interact with the IMS network, then the interaction between the IMS network and the CS network can be implemented via the eMSC.
For a UE having a variety of access methods, if the UE can use only one access method at a certain time, and it is performing a certain service such as a call with a certain access method, the UE and the network can provide a certain method not to interrupt the service being performed by the UE when the UE moves to other domains and needs to change the access method in use, and this capability is called single terminal radio voice call continuity, simplified as single radio voice call continuity (SRVCC).
FIG. 1 is a schematic diagram of the SRVCC, and it describes the signaling leg and media leg for establishing a session between the single radio voice call terminal UE-1 and the IMS terminal UE-2, and the signaling leg and media leg between the UE-1 and the UE-2 after the SRVCC happens, and to simplify the illustration and description, the S-CSCF and service continuity AS (SC AS) are combined as one entity and between them, the SIP based on IMS standard is used for communication.
Before the SRVCC happens, a session is established between the UE-1 and the UE-2, whose signaling legs are described as follows:
A102: the signaling leg between the UE-1 and the P-CSCF which communicate with each other via the SIP of the IMS; for the SC AS, this belongs to the access leg;
A104: the signaling leg between the P-CSCF and the SC AS/S-CSCF that communicate with each other via the SIP of the IMS; for the SC AS, this also belongs to the access leg;
R101: the signaling leg between the SC AS/S-CSCF and the UE-2 that communicate with each other via the SIP of the IMS, and for the SC AS, this is a remote leg;
After the SRVCC happens, both the signaling leg and the media leg between the UE-1 and the UE-2 change, wherein, the change of the signaling leg is described as follows:
A112: the signaling leg between the UE-1 and the eMSC that communicate with each other via signaling protocol of the CS domain, and for the SC AS, this is an access leg;
A114: the signaling leg between the eMSC and the SC AS/S-CSCF that communicate with each other via the SIP of the IMS, and for the SC AS, this also belongs to the access leg;
R101: the signaling leg between the SC AS/S-CSCF and the UE-2 that communicate with each other via the SIP of the IMS, and for the SC AS, this is a remote leg, and this remote leg does not change after the SRVCC happens.
FIG. 2 is an architecture diagram of the existing SRVCC, and it describes each related part in the network or each network element participating in the implementation of the SRVCC, as well as the interfaces or the connection relationship between them, and the description is as follows:
Description of the related network elements:
UE: user terminal equipment having SRVCC capability;
CS network: a network providing users with the conventional CS services;
PS network: a network providing users with the PS services;
eMSC: processing the handover request sent by the PS network, performing the inter-domain transfer of the session, relating the CS handover operation with the inter-domain transfer operation and so on;
IMS network: a network providing users with the IMS services.
Description of the related interfaces:
S202: the air interface between the UE and the CS network (briefly called the CS air interface) which is used to implement information interaction between the UE and the CS network, such as the standard Um interface;
S204: the air interface between the UE and the PS network (briefly called the PS air interface) which is used to implement information interaction between the UE and the PS network, such as the standard Uu interface;
S206: the interface between the CS network and the eMSC (also called the CS signaling interface) is different according to different network element which is specifically connected, and the interface between the eMSC and the base station subsystem is the standard Iu-CS interface, and the interface between the eMSC and other mobile switch centers is a standard interoffice signaling interface, that is, the E interface or the Nc interface;
S208: the signaling interface between the PS network and the eMSC (also called the inter-domain handover signaling interface) is to provide inter-domain handover support, and this interface is the standard Sv interface;
S210: the signaling interface between the PS network and the Internet, such as the standard SGi interface, this interface provides IP data bearer for the information interaction between the UE and the Internet, and since the IMS network is based on the Internet, it can be considered as a special kind of Internet;
S212: the signaling leg between the eMSC and the IMS network, it might be the standard I2 interface based on the SIP of the IMS between the eMSC and the IMS network, or consist of the standard Nc interface between the eMSC and the media gateway and the standard Mg interface between the media gateway and the IMS network which are connected; if it is the latter, the media gateway will translates the message at the Nc interface into the SIP message of the IMS, or vise versa; the Nc interface can be based on the SIP (Nc-SIP) or the ISDN user protocol (ISUP) (Nc-ISUP). Although both the Nc-SIP and the I2 interface are based on the SIP, the protocol only specifies the message format, and the message content depends on applications, and using I2 interface indicates that the eMSC supports the IMS related applications, while using Nc-SIP interface indicates that the eMSC supports the conventional CS related applications.
FIG. 3 is a flow chart of the method for implementing the existing SRVCC, and it describes that an IMS session is established between the UE-1 and the UE-2 so that an IMS media leg is established, and the IMS media leg consists of the media leg between the UE-1 and the PS network and the media leg between the PS network and the UE-2, and after the SRVCC happens in the UE-1, the UE-1 and the network implementation let the UE-1 use the CS domain to establish a media leg and maintain the process of original session continuity, the process comprises the following steps:
Step 301: the UE-1 sends a measurement report to the PS network that serves it via the S204 interface to the PS network, thus to report the signal intensity measurement information of the cells;
Step 302: according to the signal intensity information of each cell in the measurement report, the PS network (original PS network) that serves the UE-1 judges that the nearby CS network is more suitable to serve the UE-1, and then decides to perform handover operation;
Step 303: the corresponding network element in the original PS network, such as the mobile management entity (MME), sends a handover request to the eMSC via the S208 interface between the PS network and the eMSC, such as sending a handover request message which carries the number information of the UE-1 and the number information of the SC AS used for identifying the radio voice call continuity request and acquired by the PS network via the home subscriber server (HSS);
Step 304: the eMSC performs the standard CS handover procedure to prepare the media link resources of the target CS network;
Step 305: after completing the CS handover procedure, the eMSC sends a handover response message to the PS network via the S208 interface, such as sending a handover response message;
Step 306: after the PS network receives the handover response message, it sends a handover command message to the UE-1 via the S204 interface to notify the UE-1 to hand over to the CS domain;
Step 307: after the UE-1 receives the handover command message, it tunes the access method to access via the CS domain;
until now, a CS media leg is established between the UE-1 and the eMSC, and this media leg consists of the CS media leg between the UE-1 and the CS network and the CS media leg between the CS network and the eMSC.
The following steps follows step 303, and they have no order relationship with steps 304˜307.
Step 308: after the eMSC receives the handover request message sent from the PS network, it sends a call request to the SC AS;
the abovementioned call request is sent via the signaling leg of S212 (called interconnected signaling leg), therefore, it can be a SIP INVITE message or a ISUP initial address message (IAM); this call request carries the number information of the UE-1 and the number information of the SC AS, wherein, the number information of the SC AS is the called information, and the number information of the UE-1 is the calling information.
Step 309: the SC AS finally receives the SIP INVITE message of the IMS forwarded by the CSCF, and the SC AS judges that this is one service continuity request according to the called information, and searches out the call in process that is related to this call according to the calling information;
Step 310: the SC AS sends an IMS update request to the UE-2 via the CSCF on the signaling leg of the related call in process, such as sending an UPDATE or a reINVITE message;
Step 311: After the UE-2 receives the update request, it responds a update agreeing message, such as a “200 OK” message;
Step 312: After the SC AS receives the update agreeing message forwarded from the CSCF, it sends a call response message, such as a “200 OK” message, to the eMSC via the signaling leg of S212, and what the eMSC finally receives might be the “200 OK” message of SIP or the answer message (ANM) of ISUP;
until now, a new media leg is established between the eMSC and the UE-2, and the eMSC connects the newly established media leg with the CS media leg so as to make the UE-1 continuously communicate with the UE-2.